Ion source unit for a calutron



Aug. 19, 1958 Filed May 22, 1945 w. M. BROBECK 2,848,619 ION SOURCE UNITFOR A CALUTRON 2 Sheets-Sheet 1 INVENTOR. W/LLl/IM 5/?05EC/f w. M.BROBECK I ON SOURCE UNIT FOR A cALuTRo Aug. 19-, 1958 2 Sheets-Sheet 2Filed May 22. 1945 FIE-E INVENTOR. F 15 3 WILL/AM M. B/mmr United StatesPatent ION SOURCE UNIT FOR A CALUTRON William M. Brubeck, Oak Ridge,Tenn., assignor to the United States of America as represented by theUnited States Atomic Energy Commission Application May 22, '1945,'SerialNo. 595,194

2 Claims. (Cl. 250-413) The present invention relates to calutrons and,more specifically, to ion-producing mechanism for use in a calutron. Inone aspect the present invention comprises .an improvement uponion-producing mechanism of the type disclosed in LettersPatent of. theUnited States, No. 2,709,222, issued May 24, 1955, to Ernest 0.Lawrence. Reference is made to that patent for a full account of thenature of a calutron and the theory of its operation.

As disclosed in the patent, a calutron comprises an evacuated vesseldisposed in a powerful magnetic field and containing apparatus by meansof which a substance containing a plurality of isotopes, or comprising amixture of elements, is vaporized and then ionized. The ions areprojected as a narrow beam through'the evacuated vessel, and theinfluence of the magnetic flux causes the beam to follow a curved path.Ions of greater masstend to concentrate in that portion of thebeamadjoining the outer periphery of the curve, and ions of less 'rnass tendto congregate in that portion of the beam adjacent the inner peripheryof thecurve. By placing a collector including a plurality of pockets, inthe pathof the beam it is possible to collect-aggregates in which thedistribution of isotopes has been altered from that of'the initial, orcharge, material. For example, uranium as it normally occurs isconstituted by three of its isotopes, U U and U When a uranium compound'is'treated in a calutron the product is a pair of aggregates of whichone has been enriched with respect to U and U while the other aggregatehas been increasingly impoverished with respect to these lighterisotopes.

One of the problems encountered in the construction and operation ofcalutrons is the provision of an are capable of ionizing asatisfactorily large percentage of the vaporized charge material. It hasbeen found that when voltages high enough to generate a suitable are areemployed, usually by the combination of an electronemissive filamentarycathode and'an ionization chamber having walls maintained at positivepotential, the copious emission of electrons in undesired 'directionsfrom the filament causes rapid deterioration of adjacent elements of themechanism.

Another disadvantage found in the operation of prior calutrons has beenthe short life of the filament, a characteristic due in part to the factthat the emission of electrons from the filament has taken placethroughout its length, with the result that the electron-emissivematerial of the filament is rapidly exhausted and rendered excessivelyfrangible.

The most important object of the present invention is to improve theperformce of the ion-producing mechanism employed in a calutron.

Another object of the invention-is :to confinethe emission of electronsfrom the filament toward the ionization chamber in order to preventbombardment and erosion of adjacent members.

Another object of the invention is to increase the life of a calutronfilament by limiting the emission of electrons to .a relatively small,predetermined segment thereof.

An important feature of the invention resides in the interposition of anapertured shield plate between the ionization chamber and the filament,the shield plate and the filament being so disposed that the aperture inthe shield plate underlies only a relatively short segment thereof.

Another feature of the invention resides in the combination of theshield plate referred to in the foregoing paragraph and a second shieldplate arranged to overlie the entire filament, both shield plates beingelectrically connected to the filament, with the result that electronsare emitted from the filament only from a short segment thereof and onlyin the direction of the aperture in the lower shield plate whichoverlies the ionization chamber.

These and other objects and features of the invention will be morereadily understood and appreciated from the following detaileddescription of a preferred embodiment thereof selected for purposes ofillustration and shown in the accompanying drawings, in which:

Figure l is a view partly in side elevation and partly in cross sectionshowing a portion of a calutron constructed and arranged in accordancewith the invention,

Fig. 2 is a view in exploded isometric showing ionproducing mechanismfor a .calutron constructed and arranged in accordance with theinvention, and

Fig. 3 is a view in side elevation of the ion-producing mechanism.

No attempt is made hereinto disclose a complete calutron. For thismaterial, reference is made to the abovementioned patent of Ernest 0.Lawrence.

In Figure 1 there is shown, somewhat diagrammatically, the source end ofa calutron, or that portion of the apparatus containing theion-producing mechanism. Between the upper pole face 10 and the lowerpole face 12 of a powerful magnet there is placed a tank or vessel 14adapted to be evacuated (by means not shown) to a pressure of from 10-to 10- millimeters of mercury. The tank 14 has a side wall 16 having alarge circular aperture 18 through which passes a tubular stem 20insulated from the tank 14 and supported by means of a large heavy glassinsulator 2.6. There is also provided mechanism designated generally bythe reference character 28 by means of which the stem 20 may beuniversally adjusted with respect to the tank 14. The details of theadjusting mechanism are fully disclosed in the above-mentioned patent ofErnest 0. Lawrence.

The stem 20 terminates at'its inner end in a rod 22. Welded to the endof the rod 22 is a rectangular metal plate 32 in each corner of which isa hole receiving a screw 38 which passes through the plate 32, through atubular spacer 36 and into a hole tapped into a metal block 34. The fourscrews 38 thus serve to mount the block 34 at the end of the rod 22. Theblock 34 is provided with a vertical bore in which is mounted anelectric heating element including a core 40 of ceramic material uponwhich is wound a length of high resistance wire 42. A disk 44 overliesthe heater chamber and supports a screw 46 which engages a hole tappedin the upper end of the 'core.40 and serves to mount the heater in itsposition in the block 34. The core 40 is enlarged at its lower end asshown at 48 and provided with a dependingflange 49 which serves as aterminal block for a pair of leads 5!) and 52 which are suitablyconnected to the ends of the wire 42. vThe negativelead 50 is grounded'to therocl 22, While the positive lead 52 is connected to a rigidconductor 30 which is mounted below and parallel to the stem 20.

At the far end of the block 34 there is a second vertical bore 54 whichserves as a vaporizing chamber and charge receptacle. Before thecalutron is operated, a quantity 56 of material such as UCL; is droppedinto the bore 54. A slot 58 is milled in the block 34 and intersects thebore 54, thus providing a restricted exit passage from the chargechamber 54. A fiat rectangular plate 60 is secured to the top of theblock 34 by means of four screws 62 and is provided with an aperture 64of small diameter which is disposed to overlie the center of the slot58.

Mounted above and parallel to the stem 20 is a pair of stilt tubularconductors 24 surrounded by a pair of Water jacket tubes 66. The jackets66 extend beyond the end of the conductors 24 and terminate above theblock 34. To one of the water jackets 66 there is secured an L-shapedmetal block 68 which cooperates with a rectangular block 70. The blocks68 and 70 are provided with matched grooves, in which the end of asubstantially U-shaped filament 72 is received, and the block 70 isdrawn into clamping engagement with the block 68 by means of a pair ofscrews 71. The other of the water jackets 66 terminates in a metal block74 which has an integral plate 76 extending therefrom and overlying thefilament 72. The block 74 is cut away to form a recess on its lowerside, the recess being filled by a second block 78 having an integralplate 80 extending therefrom in position to underlie the filament 72.The block 78 and the plate 80 are drawn into engagement with the block74 by means of a pair of screws 82, and the other end of'the filament 72is clamped between the blocks 74 and 78, there being grooves (not shown)in which the end of the filament is received.

Extending through the plate 80 is a circular aperture 84 which isaxially aligned with the aperture 64 in the plate 60 and which underliesa centrally disposed segent of the filament 72.

When the device is to be operated the circuits are closed to impressabout four volts across the filament 72 and to render the arc block 34approximately 200 volts more positive than the filament 72. The plates76 and 80 are connected to the most negative side of the filament 72.The material of which the filament 72 is made is not critical, since anyelectron-emissive substance commonly used as a filament can be employedherein; tungsten and tantalum are examples of filament materials whichhave successfully been employed in calutrons.

Inasmuch as the plate 76 is at the same potential as the filament orslightly negative with respect thereto, the emission of electrons fromthe filament in the direction of the plate 76 is substantiallyeliminated. Consequently, all parts of the calutron lying above thefilament are protested against deterioration due to the bombardment byelectrons leaving the filament. Similarly the plate 80 serves to limitthe emission of electrons from the filament 72 to that portion thereofwhich lies directly over the aperture 84. It should be furtherunderstood that the action of the powerful magnetic field in which themechanism is placed serves to confine the emission of electrons toplanes parallel to the direction of the magnetic flux. The diameter ofthe aperture 84 must be sufficiently great to avoid a grid effect fromthe plate 80. In other words, the fact that the plate 80 is connected tothe most negative leg of the filament would tend to render it a gridfunctioning to limit the emission of the electrons from any portion ofthe filament. However, if the aperture 84 is made large enough the grideffect is obviated or reduced to negligible portions. The plate 60 is,of course, at the same potential as the arc block 34, or 200 volts morepositive than the filament 72. Consequently, electrons emitted from thefilament 72 are strongly attracted toward the arc block 34 and thestream of electrons- 4 which form the arc travels from the filamentthrough the apertures 84 and 64 and down through the slot 58 in the arcblock 34.

When the heating element is energized the walls of the chamber 54 areelevated in temperature until the charge material 56 is caused tovaporize. Due to the vacuum in the vessel 14 the vapor thus formed isdrawn through the slot 58 where it encounters the arc discharge. Theelectrons in the arc impinge upon the vapor molecules to causedissociation and ionization thereof, and further ionization is caused bysecondary collision.

By placing accelerating electrodes (not shown) in advance of the slot 58and applying to them a potential highly negative with respect to the arcblock 34, positive ions are withdrawn from the slot 58 or ionizationchamber and accelerated to form a dense high-velocity beam of positiveions.

It should be noted that, although the arc passes through the aperture 64in the cover plate 60, that portion of the plate 60 which immediatelysurrounds the aperture 64 will be subjected to some erosion due toelectron bombardment. However, it is an easy matter to replace the plate60 as it wears away, and it is to be noted that the arc block 34 issubstantially protected from electron bombardment by the shield plate80. As previously disclosed, the upper shield plate 76 serves to preventthe electron bombardment of all the elements of the calutron whichoverlie the filament 72. Furthermore, the lower shield plate 80 and theaperture 84 function to prevent the emission of electrons from thefilament 72 except from that portion or segment of it which directlyoverlies the aperture 84 and is therefore subject to the influence ofthe voltage drop between the filament and the arc block 34. By thusconfining the emission of electrons from the filament to a relativelyshort segment thereof, the life of the filament is substantiallyincreased and the efiectiveness of the ion-producing mechanism greatlyimproved.

Having now described and illustrated the embodiment of my invention,what I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Ion-producing mechanism which comprises a substantially U-shapedfilament, an imperforate shield plate disposed to overlie said filamentand electrically connected thereto, an apertured lower shield plateelectrically connected to said filament and disposed to underlie saidfilament except for a linear portion thereof, and a source block havingan ionization chamber disposed beneath said filament and having apassage aligned with the aperture in said lower shield plate to affordcommunication from said chamber to said filament.

2. An ion source mechanism which comprises a source block having anionization chamber, a cover plate covering one end of said chamberexcept for an aperture extending through said plate, a substantiallyU-shaped filament suspended with a linear portion thereof above theaperture in said cover plate, and a pair of shields electricallyconnected to said filament and disposed respectively above and belowsaid filament, the lower shield having an aperture aligned with theaperture in said cover plate.

References Cited in the file of this patent UNITED STATES PATENTS1,285,215 Kelly Nov. 19, 1918 1,878,338 Smith Sept. 20, 1932 1,467Bleakney Nov. 12, 1940 2,374,205 Hoskins Apr. 24, 1945 2,378,962Washburn June 26, 1945

